Many consumer electronic devices are now being built with touch sensitive screens, for use with finger or stylus touch user inputs. These devices range from small screen devices such as mobile phones and car entertainment systems, to mid-size screen devices such as notebook computers, to large screen devices such as check-in stations at airports.
Most conventional touch screen systems are based on resistive or capacitive layers. Such systems are not versatile enough to offer an all-encompassing solution, as they are not easily scalable.
Reference is made to FIG. 1, which is a prior art illustration of a conventional touch screen system. Such systems include an LCD display surface 606, a resistive or capacitive overlay 801 that is placed over the LCD surface, and a controller integrated circuit (IC) 701 that connects to the overlay and converts inputs from the overlay to meaningful signals. A host device (not shown), such as a computer, receives the signals from controller IC 701, and a device driver or such other program interprets the signals to detect a touch-based input such as a key press or scroll movement.
Reference is made to FIG. 2, which is a prior art illustration of a conventional resistive touch screen. Shown in FIG. 2 are conductive and resistive layers 802 separated by thin spaces. A PET film 803 overlays a top circuit layer 804, which overlays a conductive coating 806. Similarly, a conductive coating 807 with spacer dots 808 overlays a bottom circuit layer 805, which overlays a glass layer 607. When a pointer 900, such as a finger or a stylus, touches the screen, a contact is created between resistive layers, closing a switch. A controller 701 determines the current between layers to derive the position of the touch point.
Advantages of resistive touch screens are their low cost, low power consumption and stylus support.
A disadvantage of resistive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is that pressure is required for touch detection; i.e., a pointer that touches the screen without sufficient pressure goes undetected. As a consequence, resistive touch screens do not detect finger touches well. Another disadvantage is that resistive touch screens are generally unreadable in direct sunlight. Another disadvantage is that resistive touch screens are sensitive to scratches. Yet another disadvantage is that resistive touch screens are unable to discern that two or more pointers are touching the screen simultaneously, referred to as “multi-touch”.
Reference is made to FIG. 3, which is a prior art illustration of a conventional surface capacitive touch screen. Shown in FIG. 3 is a touch surface 809 overlaying a coated glass substrate 810. Two sides of a glass 811 are coated with a uniform conductive indium tin oxide (ITO) coating 812. In addition, a silicon dioxide hard coating 813 is coated on the front side of one of the ITO coating layers 812. Electrodes 814 are attached at the four corners of the glass, for generating an electric current. A pointer 900, such as a finger or a stylus, touches the screen, and draws a small amount of current to the point of contact. A controller 701 then determines the location of the touch point based on the proportions of current passing through the four electrodes.
Advantages of surface capacitive touch screens are finger touch support and a durable surface.
A disadvantage of surface capacitive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is a limited temperature range for operation. Another disadvantage is a limited capture speed of pointer movements, due to the capacitive nature of the touch screens. Another disadvantage is that surface capacitive touch screens are susceptible to radio frequency (RF) interference and electromagnetic (EM) interference. Another disadvantage is that the accuracy of touch location determination depends on the capacitance. Another disadvantage is that surface capacitive touch screens cannot be used with gloves. Another disadvantage is that surface capacitive touch screens require a large screen border. As a consequence, surface capacitive touch screens cannot be used with small screen devices. Yet another disadvantage is that surface capacitive touch screens are unable to discern a multi-touch.
Reference is made to FIG. 4, which is a prior art illustration of a conventional projected capacitive touch screen. Shown in FIG. 4 are etched ITO layers 815 that form multiple horizontal (x-axis) and vertical (y-axis) electrodes. Etched layers 815 include outer hard coat layers 816 and 817, an x-axis electrode pattern 818, a y-axis electrode pattern 819, and an ITO glass 820 in the middle. AC signals 702 drive the electrodes on one axis, and the response through the screen loops back via the electrodes on the other axis. Location of a pointer 900 touching the screen is determined based on the signal level changes 703 between the horizontal and vertical electrodes.
Advantages of projective capacitive touch screens are finger multi-touch detection and a durable surface.
A disadvantage of projected capacitive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is their high cost. Another disadvantage is a limited temperature range for operation. Another disadvantage is a limited capture speed, due to the capacitive nature of the touch screens. Another disadvantage is a limited screen size, typically less than 5″. Another disadvantage is that surface capacitive touch screens are susceptible to RF interference and EM interference. Yet another disadvantage is that the accuracy of touch location determination depends on the capacitance.
Conventional optical touch screens project light beams from one edge of the screen, over and across the screen surface to where photo detectors detect the uninterrupted beams. Touches are detected when an object placed on the screen blocks one or more of the projected light beams, and some of the photo detectors do not detect the expected light.
A disadvantage of conventional optical touch screens is that they require a raised bezel around the screen in order to project the light beams across the screen. This requirement is incompatible with some product designs that require a completely flat upper surface with the edges of the device being flush with the screen surface.
Another disadvantage of conventional optical touch screens is an artifact known as “ghosting”. Ghosting is manifested when a pointer such as a finger completely blocks a light beam, and a second pointer situated inside the shadow of the blocked beam goes undetected, since the second pointer does not affect the amount of light that reaches the detector.
It would thus be beneficial to provide touch screens that overcome the disadvantages of conventional resistive and capacitive touch screens described above, while enabling flush device designs and detecting multiple objects in a single beam's path.